I agree with @Scott, that “touch” is a slippery slope for robot-human interaction, and it should be approached cautiously.
I believe that touch is something that is very personal and also context-dependent. Touch requires permission, and so, before robots go around touching humans, robots need the ability to ask for and acknowledge consent. They need to ability to resolve the difference between yes and no. Robots also need the ability to understand that consent in one context, does not extrapolate to mean that consent has been granted for touching in all contexts. Similarly, before humans go around touching robots, robots need to be equipped with the ability to give & revoke consent and possess some level of understanding of what that means.
This article from HRI 2011 is the one that comes to my mind when I think about robots touching humans: (https://smartech.gatech.edu/bitstream/handle/1853/41834/0000057267-Touched%20By%20a%20Robot%20An%20Investigation%20of%20Subjective%20Responses%20to%20Robot-Initiated%20Touch.pdf)
I imagine that in the future, robots will be similar to humans in many facets. In modern society, there are laws that dictate (socially) acceptable human-human touching. So too, in the future, maybe robot-robot and robot-human touching will be governed by similar rules.
And then there are individual differences to consider also. Consider animals as examples. Some cats are friendly and like to be touched by humans. Other cats prefer to be left alone and do not want humans touching them. I believe that in the future, there will also be variance in robot preference about being touched by humans. Each robot will regard being touched differently, and so, it won’t be acceptable for humans (or robots) to walk around randomly touching robots.